Upon activation, platelets from the primary hemostatic plug at sites of vascular injury. In addition, activated platelets provide an efficient pro- coagulant surface for the subsequent activation of the coagulation cascade. The transbilayer movement of anionic phospholipids from the inner to the outer leaflet of the platelet membrane is a major determinant of pro-coagulant activity. This is accompanied by the release of procoagulant membrane particles called microvesicles. Individuals with impaired generation of platelet procoagulant activity have a life-long bleeding diathesis (Scott syndrome) Our studies show that shear stress, such as that which occurs at a site of atherosclerotic narrowing of arteries, is a potent stimulus for the development of platelet procoagulant activity and microvesiculation. Moreover, thrombin accentuates the shear stress- induced anionic phospholipid exposure and microvesiculation. The microvesicles adhere to matrix components, provide a binding site for platelet adhesion, and localize to the site of vascular injury in vivo. Microvesicles bind beta2-glycoprotein I, an anionic phospholipid-binding plasma protein, which promotes its clearance by macrophages. The presence of autoantibodies to beta2-glycoprotein I, as well as inherited mutations in beta2-glycoprotein I (which impair its philosophical- binding) are associated with increased incidence of thrombosis. In this proposal, we will investigate the role of shear stress-induced platelet procoagulant activity on thrombus formation. The specific aims of this proposal are: (1) To define the signal transduction pathways leading to shear stress-induced platelet pro-coagulant activity. We will test the hypothesis that signal transduction that signal transduction through ITAM-containing proteins and Syk leads to anionic phospholipid exposure and microvesiculation. (2) To assess, under shear stress, the development of platelet pro-coagulant activity and its role in the stability of thrombus. The hypothesis to be tested: Platelets develop procoagulant activity following adhesion on von Willebrand factor and/or a collagen surface and this activity on adherent platelets plays a significant role in thrombus stability. (3) To define the role of beta2-glycoprotein I in the clearance of platelet-derived microvesicles from the circulation. The hypothesis to be tested: beta2-glycoprotein I binds to anionic phospholipid on the surface of platelet-derived microvesicles and mediates their clearance by macrophages. The corollary of this is that deficiency of beta2-glycoprotein I function will lead to a hypercoagulable state.